Has anyone investigated the possibility of replacing the 500 W heater with a similar heater of 700 W or so?
It might be a simpler way to address the needs or many users for more heating capacity, such as those of us in the warmer parts of the country. Using a capacity of ~700 W would certainly reduce the need for a PID controller. And if a heater with similar dimensions could be found, it would avoid tinkering with wiring, sheet metal modifications and other issues associated with a second 500 W or a single 900 W unit.